Three major herbaceous groups, the Asteraceae, Amaranthaceael Chenopodiaceae, and Poaceae, shed abundant pollen in coastal southern Texas during late summer to fall. The grasses, and to a lesser extent Parthenium (Asteraceae), also release pollen in the spring, giving marked bimodal distributions. These aeropollen patterns were consistent between 1987 and 1989. Although environmental conditions and palynological features are important in understanding aeropollen variabilities, differing inherent photoperiodic response of wind pollinated taxa in the spring and fall, and to a more limited extent during summer and winter, are of primary relevance at Corpus Christi, and indeed elsewhere, in the initiation and cessation of flowering, and thus of the release of ambient pollen. To comprehend photoperiodicity is to understand the fundamental patterns of aeropollen occurrences year after year.